Optical networks, which use optical fiber as a transmission medium, are well known for use in transmission of digital data signals. To make efficient use of a single optical fiber, many unique data signals may be transmitted over the same fiber so long as each data signal modulates an optical signal with a wavelength different from the other optical signals on the same fiber. This transmission scheme is called Wavelength Division Multiplexing (WDM). When the wavelengths of the different optical signals are only marginally different from one another, the transmission scheme may be called Dense Wavelength Division Multiplexing (DWDM). In a network using DWDM, two elements connected by a single physical link (optical fiber) may communicate using a number of signal channels, where each signal channel is an optical signal with a distinct wavelength. However, each signal channel may not be independent of the other signal channels transmitted on the same fiber, even though the other signal channels are transmitted at different wavelengths. An effect called Stimulated Raman Scattering (SRS) may allow crosstalk from one or more signal channels to cause changes (amplification or attenuation) to occur in a signal channel of interest.
SRS is an important non-linear effect in optical fibers named for the scientist who discovered a phenomenon in the scattering of light, called the Raman Effect, in 1928. Raman scattering occurs in a silica fiber when light propagates through the fiber. Stimulated Raman Scattering is an inelastic scattering process in which an incident photon loses its energy to create another photon of reduced energy at a lower frequency. That is, energy of a first signal at one wavelength may effect a second signal at a longer wavelength. SRS is often portrayed as a plot of Raman gain, gR, as a function of frequency (or wavelength). In such a plot, the magnitude of the Raman gain may be shown to vary over a Raman bandwidth (i.e., the bandwidth over which gR is non-zero).
As WDM systems move towards densely packed wavelengths in DWDM and related high channel count, SRS becomes a critical factor in system performance. Accordingly, modeling SRS accurately has been a research topic recently.
It has been shown that a WDM system is optimized when all channels have equal power or, alternatively, Optical Signal to Noise Ratio (OSNR) at the output of the fiber. Several schemes have been proposed for equalizing channel output power. In one scheme, disclosed by Chraplyvy in U.S. Pat. No. 5,225,922, issued Jul. 6, 1993, the output power of each channel is measured at the receiver. Based on these measurements, adjustments are made to the optical input signal power for each channel. In another scheme, disclosed by Khaleghi, et al, in U.S. Pat. No. 6,040,933, issued Mar. 21, 2000, optical power measurements of each channel are taken at inputs of optical amplifiers along the transmission path of the system. The channels are equalized by adjusting the optical input signal powers of each channel based on the measurements. In each of these schemes, there is a requirement for accurate power measurement and a feedback path over which instructions may be received for making adjustments based on the measurements. However, neither scheme may be used in the absence of a feedback path. Further, accurate optical power measurement generally involves expensive hardware. It would be desirable, then, to optimize a WDM system by equalizing output power for each channel in the system even in the absence of a feedback path.